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Microbial analysis of commercially available US Queso Fresco
Journal of Dairy Science ( IF 3.5 ) Pub Date : 2018-07-13 , DOI: 10.3168/jds.2017-14037
Maxwell J. Holle , Luis A. Ibarra-Sánchez , Xiaoji Liu , Matthew J. Stasiewicz , Michael J. Miller

Queso Fresco (QF), a fresh Hispanic-style cheese, is often associated with Listeria monocytogenes outbreaks and recalls. Queso Fresco's susceptibility to bacterial contamination is partially due to its high pH and moisture content as well as Listeria's tolerance for the salt content typical for QF. Nine different brands of US QF, 2 packages from 4 different lots (to account for temporal variability), were sampled. The pH, salt content, and moisture content were analyzed in addition to microbial testing including yeasts and molds, coliforms, lactic acid bacteria enumeration, and L. monocytogenes counts. The cheeses were also inoculated with a cocktail of 5 food and human isolates of food-borne outbreak-associated Listeria monocytogenes strains to evaluate how the differences between brands influenced Listeria growth. Three of the cheeses underwent additional genus-level microbial analysis using extracted 16S rDNA, allowing for phylogenetic analysis between bacterial taxa including diversity and relative abundance. We found little variation between the sampled QF pH (range = 6.62–6.86), salt content (1.53–2.01%), and moisture content (43.90–54.50%). Yeasts and molds were below the detection limit of enumeration in all of the cheeses and coliforms were below the detection limit across the first 3 lots, but were detected at varying levels in the fourth lot (>3.0 most probable number/g) for 3 of the brands. Listeria monocytogenes was not isolated after enrichment in any of the samples. All cheeses tested positive for the presence of lactic acid bacteria, with only 1 of the cheeses being labeled as produced with added cultures having substantial counts. Fourteen days after inoculation with L. monocytogenes, at least 2.5 log10 cfu/g of growth was found for all QF brands stored at 4°C. Microbial genus analysis showed that, among the 3 brands, the microbial community was more similar within brand than when compared with the other 2 brands. Thermus, Anoxybacillus, and Streptococcus accounted for the dominant genera of brands A, B, and C, respectively. These variations within the microbial community may account for sensory differences and help manufacturers determine quality control consistency more readily than culture-based methods.



中文翻译:

市售美国Queso Fresco的微生物分析

Queso Fresco(QF)是一种新鲜的西班牙风味奶酪,通常与单核细胞增多性李斯特菌的爆发和召回有关。Queso Fresco对细菌污染的敏感性部分归因于其高pH和水分含量,以及李斯特菌对QF典型盐分的耐受性。抽样了9种不同的美国QF品牌,分别来自4个批次的2个包装(考虑了时间变化)。除了微生物测试外,还分析了pH,盐含量和水分含量,包括酵母和霉菌,大肠菌,乳酸菌计数和单核细胞增生李斯特菌计数。奶酪还接种了5种食物和与食源性暴发相关李斯特菌的食源性分离株的混合物应变以评估品牌之间的差异如何影响李斯特菌的生长。使用提取的16S rDNA对其中的三种奶酪进行了其他属级微生物分析,从而可以对细菌分类群之间的系统发育进行分析,包括多样性和相对丰度。我们发现采样的QF pH(范围= 6.62–6.86),盐含量(1.53–2.01%)和水分含量(43.90–54.50%)之间几乎没有变化。酵母和霉菌均低于所有干酪的计数检测限,大肠菌在前三批中均低于检出限,但在第四批中,有3种中检出的霉菌水平不同(> 3.0最可能数/克)。品牌。李斯特菌富集任何样品后均未分离。所有奶酪的乳酸菌含量均呈阳性,只有一种奶酪被标记为添加有大量细菌的培养物。接种单核细胞增生李斯特氏菌14天后,发现所有在4°C保存的QF品牌产品的生长至少2.5 log10 cfu / g。微生物属分析表明,与其他两个品牌相比,在三个品牌中,品牌内的微生物群落更为相似。ThermusAnoxybacillus链球菌分别占品牌A,B和C的主导属。与基于培养的方法相比,微生物群落中的这些变化可能会导致感觉差异,并帮助制造商更轻松地确定质量控制的一致性。

更新日期:2018-07-14
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